In the field of flash photography there is often a need to change the scene coverage angle of the illumination to match the scene coverage angle of a zoom lens. There is an advantage in doing this because the range, or distance of proper exposure can be extended for the telephoto end of the zoom. In addition to this, the scene coverage angle can be increased to illuminate the area needed for the wide angle end of the zoom. Electronic flash apparatus generally includes a flash lamp and a flash reflector for directing the light from the flash lamp into a beam having a particular scene coverage angle. The need to change the coverage of the flash has long been recognized. Matching the light pattern to the photographic field and using a means of auto exposure is one way to conserve on energy used per photograph. This is important because in most cases the flash is portable and battery powered. By minimizing the energy used per photo, the recycle time to charge up for the next flash is shorter and more photos can be taken per battery charge. It is also well known that the flash distance for a telephoto lens can be increased by concentrating the light into the smaller scene coverage angle of this lens. With these benefits in mind, a number of techniques have been developed to change the scene coverage angle of a photographic flash apparatus.
One way to change the scene coverage angle of the flash is by moving the flash lamp along the optical axis of the flash reflector. As the lamp is moved away from the back of the reflector the pattern of light is spread to cover a larger area. A disadvantage to this method is that the flash lamp must be moved in a very accurate manner. If the mechanism allows the flash lamp to move off the optical axis of the reflector, then the pattern will also shift as a function of the location of the flash lamp with respect to the reflector. Flash reflectors generally have end plates which must have a slots to allow the lamp to move. This reduces the area of the end plates and thus their reflection efficiency.
Another method of changing the scene coverage angle is by moving a lens in front of the reflector along its optical axis. This method is very effective, but it requires sufficient clearance in front of the reflector for the lens to travel along its optical axis and some mechanism for supporting and moving the lens.
A still further method of changing the scene coverage angle of a flash by changing the shape of the reflector. This provides an opportunity to change the distribution of light within the scene coverage angle in addition to changing the scene coverage angle itself. While this is a valuable feature, the mechanism to do this can be complicated and sensitive to operate. Depending on how much change is needed, the volume of the reflector may change substantially.